The present invention relates to a railway signalling system.
It is well known that the headway-critical areas of a metro railway are at stations, turn-arounds and junctions. Here, the minimum permitted-separations between normal-running trains are constrained by station dwell periods, the time required for braking and accelerating, and the time for points to be reset. Conventional fixed-block systems (such as track circuit-based fixed block systems) constrain the separations further because of the time required for trains to clear block boundaries. Fixed block systems also force trains to brake prematurely for track obstacles (stationary trains, junctions with conflicting routes set, etc.). The braking, rather than being a smooth curve, consists of a succession of stepped-down curves.
Metro authorities, facing ever increasing passenger demand, are looking for methods of increasing the maximum train throughput, thereby increasing the offered capacity for the same journey times and dwell periods. A method which fulfills this aim, whilst not incurring considerable cost and effort in modifying existing track circuit layouts, is very desirable. In any case, track circuit technology already works close to its practical limit in terms of achievable headway.
A typical track circuit-based system is illustrated in FIG. 1, which shows plots of speed against distance of a train in relation to a platform 2. The curves in full lines represent typical "service braking" and the curves in broken lines represent typical "emergency braking" profiles. References B1-B5 designate block sections of a track T, and reference numerals 3 designate block section boundaries. Whilst train 1 is stationary at the platform 2, the track circuit codes established in the block sections immediately behind could be as shown. For example, in block section B1, the code is denoted by "80/60". This means that the maximum speed permitted in the block section is 80 km/hr, and the target speed is 60 km/hr. The target speed is the speed for which the driver or an automatic driving system should aim to achieve before leaving the block section. If the train enters block section B2 with a speed greater than 60 km/hr (allowing for equipment tolerances) then the emergency brakes should be applied by a train-borne automatic train protection (ATP) system. The same would be true for block section B2 if the train, having reduced its speed to 60 km/hr, failed to brake to the new target speed of 40 km/hr. (N.B. these speed values are notional values, and are set according to the characteristics of a particular railway). The block section immediately behind the stationary train 1 (or other "obstacle") is coded "0/0". This block section acts as an emergency "overlap" distance. In the worst case, a train braking under emergency conditions would come to rest with its nose at the end of this block section.
FIG. 2 shows how the track circuit codes are updated as a train leaves the station. It also shows how the minimum headway is set according to how close the approaching train can approach the departing train without having to brake for restrictive track circuit codes.
In effect, a train under track circuit control is only "aware" of the position of the train ahead as the latter clears block section boundaries. The following train has no knowledge of the position of the train ahead within a block section. This is reflected in the stepped nature of the limit of movement authority which, as shown in FIG. 2, corresponds to the target point for the following train for normal service braking.
In terms of headway performance, track circuit arrangements suffer from the following disadvantages:
The position of a train is defined only by track circuit occupancy. For typical metro applications, this gives a minimum resolution no better than about 100 meters, depending on the number of track circuit codes available. PA1 The minimum separation between trains is governed by the maximum permitted train speed and not by a train's actual speed. This means that slower moving trains take longer to clear block sections, thereby impeding the progress of a train behind. Furthermore, it means that the headway performance of lower performance rolling stock is constrained by the track circuit requirements for the highest performance rolling stock. PA1 (i) To permit trains to move through headway-critical zones of an urban passenger railway (metro) with safe distances of separation that are shorter than those achievable using conventional fixed block systems of protection. This increases the passenger-carrying capacity of the railway for the same inter-station journey times, dwell periods and rolling stock performance. PA1 (ii) To permit an existing fixed block system, such as a fixed block track circuit system, to maintain safe distances of train separation over areas that are not headway-critical. This will usually be inter-station sections where, under normal headway conditions, train spacings are far greater than in headway-critical zones. PA1 (iii) To permit the protection of train movements in headway-critical areas to revert to fixed block control, such as a fixed block track circuit control, when a moving block control system shuts down because of a failure. PA1 (iv) To increase the flexibility of control over trains approaching stations; for example, to control the approach speed in order to minimize the headway at the expense of inter-station journey time. PA1 (v) To permit energy-saving coasting control to be implemented without degrading the achievable headway. Such a facility would be particularly beneficial during an oil crisis, for example, when the metro authority may wish to implement peak-hour coasting over a long-term period, but not suffer loss of offered capacity.
Certain objectives of a railway signalling system which the present invention aims to enable to be achieved are set out below:
U.S. Pat. No. 4,166,599 discloses a system in which, in a fixed block system, there is communication between vehicles via a communication channel so that a vehicle is informed of the next adjacent downstream occupied block section, but there is no back-up control if the communication channel breaks down.
EP-A-0 341 826 discloses a railway signalling system comprising both fixed and moving block control in which a transmit-only zone exists on the departure side of a platform and a receive-only zone exists on the approach side. The transmission is direct from the departing train to the one approaching. Also, the system described in EP-A-0 341 826 relies on the fixed block system to prevent a further train from entering the communication area when one is already receiving messages.